(1) Field of the Invention
The present invention relates to a hydrophilic hollow fiber microporous membrane suitable for the removal of fine particles and bacteria contained in water or an aqueous solution, and a process for the preparation of this membrane.
More specifically, the membrane of the present invention is advantageously used for the production of germ-free water, the separation of valuable components (e.g., enzymes) from cells in a fermentation bulk liquid, and the removal of iron cladding from nuclear power condensed water.
(2) Description of the Related Art
A variety of microporous membranes having fine pores of the micron-to-submicron order, ranging from membranes having a laboratory size to membranes having an industrial size, are marketed as microfilters.
These membranes are roughly divided into plain membranes (film-shaped membranes) and hollow fiber-shaped membranes. Hollow yarn-shaped membranes are superior to plain membranes in that cross-flow and back washing is possible, and when modules are constructed, the membrane pack density can be high.
As the material of the hollow fiber membrane, there can be mentioned hydrophobic polymers represented by polyolefins and hydrophilic polymers represented by cellulose derivatives.
A polyolefin microporous membrane has an excellent chemical resistance and a high strength, but when water or an aqueous solution is treated, since the membrane per se is hydrophobic, a pretreatment operation of passing a water-soluble organic solvent having a low surface tension, such as ethanol, in advance, and substituting the solvent with water is carried out. Nevertheless, even if this pretreatment is performed, bubbles become entangled in the membrane during the operation or while stored, and if the membrane is dried, water can not pass through such bubble-entangled portions, resulting in a reduction of the water permeation rate.
The foregoing problems do not arise in a microporous membrane composed of a hydrophilic polymer, such as a cellulose type microporous membrane, but this microporous membrane has poor chemical resistance, such as acid or alkali resistance, and has poor mechanical strength.
As a means for solving the foregoing problems and providing a microporous membrane having an excellent hydrophilic property, chemical resistance, and mechanical strength, various methods for modifying the surfaces of a hydrophobic microporous membrane including the surfaces defining the pores have been investigated.
As the surface-modifying method, there can be mentioned a chemical modification method for chemically modifying the surface with a chemical, a coating method for dissolving a hydrophilic polymer in a solvent therefore, dipping a porous membrane in the solution, and drying the membrane to impart a hydrophilic property to the surfaces of the pores, a plasma method, and a grafting method.
For Example, Japanese Examined Patent Publication No. 61-2,100 discloses a chemical modification method in which a polyolefin porous membrane is sulfonated; Japanese Unexamined Patent Publication No. 61-125,408 discloses a coating method in which an ethylene/vinyl acetate copolymer is coated and saponified; Japanese Examined Patent Publication No. 62-19,208 discloses a coating method using polyethylene glycol or other hydrophilic polymeric materials; Japanese Unexamined Patent Publication No. 61-86,908 discloses a plasma method in which a hollow fiber microporous membrane is treated by low-temperature plasma; and Japanese Examined Patent Publication No. 56-44,098 discloses a grafting method in which a thin porous film of a high polymer, different from a hollow fiber microporous membrane, is modified by radiation grafting.
The chemical modification method has a problem, however, in that the mechanical strength is lowered by a deterioration of the membrane material, and the coating method has a problem in that, since the membrane material is not chemically bonded to the coated hydrophilic polymer, the coating agent is dissolved therefrom. Furthermore, in the plasma method, it is difficult to uniformly treat the surfaces of pores of the porous membrane, and even if the porous membrane can be rendered hydrophilic by the plasma method, the water permeation pressure is 2.5 kg/cm.sup.2 or more, as shown in the examples of Japanese Unexamined Patent Publication No. 61-86,908, and the hydrophilic property is still unsatisfactory.
In the modification by the grafting method, as disclosed in the examples of Japanese Examined Patent Publication No. 56-44,098, it is known that, if a hydrophilic monomer such as acrylic acid or vinylpyridine is grafted, the water permeation performance is drastically reduced to a value of from one of scores to one of hundreds, and accordingly, the grafting method is still unsatisfactory as a surface-modifying method for improving the surface characteristics alone while maintaining the inherent characteristics of the material.
As seen from the foregoing description, although a hollow fiber microporous membrane having satisfactory characteristics such as hydrophilic property, chemical resistance, mechanical strength, water permeability, and durability is strongly desired, such a membrane has not been previously developed.